Precoating of aluminum can sheet

ABSTRACT

The successive steps of coating aluminum can sheet with a resin capable of withstanding subsequent drawing and ironing, curing the resin coated on the sheet; drawing the sheet, for example, into the shape of a shallow can, and ironing the shallow can into a deep can of beverage size.

D United States Patent 11 1 1111 3,832,962 Rolles Sept. 3, 1974 [54]PRECOATING 0F ALUMINUM CAN SHEET 3,206,848 9/1965 Rentmeester 1 13/120 A3,268,620 8/1966 [75] Inventor: New Kensmgton, 3,360,157 12/1967 Bolt eta1. 113 120 A 73 Assigneez l i Company f America 3,497,466 2/1970Markulin et a1. 220/64 Pittsburgh Pa. 3,539,480 11/1970 Groff et a1.220/64 3,577,753 5/1971 Shah et a1. 113/120 [22] Filed: July 30, 1973[21] Appl. No.: 384,039 Primary ExaminerRichard J. Herbst Related USApplication Data Attorney, Agent, or FirmJohn P. Taylor [63]Continuation-impart of Ser. No. 174,136, Aug. 23, i

1971, abandoned, and a continuation of Ser. No. 805,829, March 10, 1969,abandoned. [57] ABSTRACT The successive steps of coating aluminum cansheet 2% 113/120 with a resin capable of withstanding subsequent draw-72/46 ing and ironing, curing the resin coated on the sheet; 1 o earcdrawing the sheet, for example, into the shape of a shallow can, andironing the shallow can into a deep [56] References Cited can of beverae Size UNITED STATES PATENTS g 2,856,094 10/1958 Gloyer et a1. 220/64 6Claims, N0 Drawings PRECOATING OF ALUMINUM CAN SHEET CROSS REFERENCE TORELATED APPLICATION This application is a continuation-in-part ofapplication Ser. No. 174,136 filed Aug. 23, 1971, now abandoned, as astreamlined continuation of application Ser. No. 805,829 filed Mar. 10,1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the manufacture ofdrawn and ironed containers from precoated aluminum sheet. Moreparticularly, it relates to a process in which a particular coating isapplied to and cured on aluminum sheet prior to drawing and ironing.'Theterm aluminum as used herein includes aluminum and its alloys containingat least 50 percent by weight aluminum US. Pat. No. 3,206,848 disclosesprecoating of aluminum sheet with vinyl organosols, solution vinyls,vinyl phenolics and epoxy phenolics prior to drawing the coated sheetinto a shallow cup, heating the shallow cup to a temperature of around275380 F which is below the initial hardening temperature of the coatingfor 1-20 minutes to relieve the stresses in the coating and the aluminumalloy and thereby providing greater coating elongation and minimizingthe possibility of subsequent fracture, cooling and then drawing againto a depth of about 2% inches. According to the patentee, additionaldrawing operations may be performed provided there are baking stepsbetween each of the draws. Such a baking operation tends to result inpossible mar ring or decomposition of the coating as well as reducingthe tensile properties of the side wall of the shallow cup formed.Therefore, development of a process which would minimize possiblecoating and metal partial anneal and also permit formation of a deep cupfrom precoated sheet represents a highly desirable result.

SUMMARY OF THE INVENTION It is accordingly one object of this inventionto provide a method of forming a deep article such as'a beveragecontainer from precoated aluminum sheet. A further object is to providea method of minimizing the formation of cracks and fractures in thecoating of precoated aluminum articles such as cans of beverage depthduring or subsequent to the drawing and ironing steps used in formingsuch articles. Further objects will be apparent from the description andclaims which follow.

DESCRIPTION OF THE INVENTION In its broader aspects, my inventioninvolves precoating aluminum sheet with a preselected compositioncapable of withstanding substantial deformation in subsequent drawingand ironing steps without substantial fracture or exfoliation, drawingthe cured coated sheet into shallow cup and then ironing theshallow cupto a can of beverage can depth. I have found surprisingly that byironing the precoated shallow cup to a deeper can size, I produce a canthat retains a smooth, substantially unfractured coating without anyheating above room temperature of about -30 C except when the coating isthermosetting and requires heating for curing before the drawing step asapplied to the sheet. This is particularly suprising in view of the factthat it has heretofore been generally believed that heating prior to orduring forming is necessary to permit sufficient elonga- I tion of themetal and flexibility for the coating to assume the shape of the caninto which the blank sheet is formed without a substantial amount ofcracking or removal of coating, especially when ironing of the side wallis involved rather than metal drawing.

The composition which I use for precoating according to my inventionmust be able to accept the strain of deformation resulting from drawingfollowed by ironing, yet remain integral without undergoing substantialfracture or exfoliation. Resins which I have found to have such aproperty and be especially useful according to my invention are certainspecific vinyl or epoxy resin types which apparently allow the requireddegree of coating elongation or coating flow (without heating) duringthe drawing and ironing steps which follow the curing of the resin onthe aluminum sheet.

Representative vinyl resins useful according to my invention includethose having a vinyl chloride content of 81 to 97% and a vinyl acetatecontent of 3 to 14%. Such vinyl polymers may be modified withappropriate dibasic acids such as maleic acid or anhydride or vinylalcohol. The foregoing resins may also be modified with plasticizers,stabililizers, solvating agents or pigments as desired. Whencross-linking is desired prior to coating, it can be obtained bymodifying the vinyl polymers, particularly those containing freehydroxyl or carboxyl groups, with a resin which is capable ofcrosslinking therewith. For example, urea-formaldehyde, triazine,melamine-formaldehyde, epoxy resins, phenolic resins or combinationsthereof may be used for crosslinking and to obtain desired physical andchemical properties.

the cross-linking should not exceed about of the theoreticalequivalency. This helps maintain elongation of film under stress andcompression and prevents excessive brittleness.

The coating resins useful according to my invention may be applied bydirect or reverse roller coat application, by spraying or by curtaincoating. Conventional catalysts may also be used to facilitate curing.When solvents are used, heating is desirable to assist solventevaporation and cross-linking. A lubricant such as paraffin wax,low-molecular weight polyethylene, or the like, may be incorporated inthe coating resin. Preferred range of lubricant concentration is from 1%to 5% based on resin solids composition. The lubricant may be applied asa separate step by dispersion in a suitable liquid. It may be applied byroller coating, spraying or cascading. The preferred range of lubricanton the coating surface is from '5 to 40 mg/ft of coated surface.

The ironing step according to my invention is capable of reducing themetal thickness from as great as 17 mils to as little as 4 mils. Thealuminum which I use may be either of the hard (H) or annealed (O)temper, and the resin coating may be applied to one or both sides of thesheet.

If desired, before the resin is applied, the aluminum sheet may beprecleaned and be subjected to a conversion coating treatment. Analkaline or acid-base cleaner may be used. It may contain sodium orpotassium hydroxide, tetrasodium pyrophosphate or other effectivealkaline salts combined with conventional wetting agents, buffers andchelating chemicals. If of acid base, the cleaner may contain nitric orsulfuric acid, for example. The conversion coating may be anelectrochemically produced aluminum oxide film such as produced bysulfuric acid anodizing or a complex metal oxide film such as producedby chemical conversion coating of the chromium-phosphate type, forexample, as described in U.S. Pat. No. 2,438,877. The anodicallyproduced coating may be conventionally sealed in boiling, de-ionizedwater but is preferably left unsealed to obtain maximum adhesion of thesubsequently applied resin coating.

The drawing prior to ironing may be in more than one step. For example,a reverse drawing step may be used.

The following examples are illustrative of my invention:

EXAMPLE 1 To illustrate the prior art, chromium-phosphateconversion-coated 3004-1119 0.0145 aluminum was used for this example.Six-inch strip was cleaned with an alkaline soidum pyrophosphate cleanerand coated with a chromium-phosphate conversion coating solution toobtain a film weight of 14 mg/ft The strip was blanked, drawn, redrawnand sized, and then ironed into beverage cans. The finish was bright onthe outer wall and wall thickness was equivalent to uncoated cans (about0.0055). The cans were solvent cleaned to remove excess lubricant.

To study metal flow and conversion coating uniformity, X-rayfluorescence and microprobe surface studies were made on the bottom andside wall exterior and interior of the 21 1-12 oz. container. Thesetests showed very uniform metal and surface flow. Distribution ofconversion coating was substantially uniform when comparing variationsof chromium between the unworked bottom and the worked side wall. Therewas a 3:1 surface conversion coating reduction. There were littledifferences between metal flow on the mandrel side (interior) and thering side (exterior) of the can. To assess the surface optically forconversion coating uniformity, cans were heated to 800 F for 15 minutesto develop color in the conversion coating. This converted thetransparent chromium-phosphate film to a strongly colored layer. Coloruniformity on the side wall was excellent and in agreement with theshade attained on surfaces with similar conversion coating thickness notmechanically worked. Microscopic studies showed that the surface of thepretreated drawn and ironed can was somewhat more broken up than thesurface of the drawn and ironed can post-treated withchromium-phosphate;

EXAMPLE 2 Aluminum Alloy 3004-1-119, 0.0145 was cleaned andpre-treatedwith a chromium-phosphate conversion coating and one side ofthis sheet was subsequently coated with a solution vinyl coating. Thevinyl coating consisted of a mixture of a vinyl chloride resin havingabout 87% vinyl chloride content and 13% vinyl acetate content and asecond resin having 86% vinyl chloride content, 13% vinyl acetatecontent and 1% maleic acid content. Equal portions of each resin wereblended with about 20% dioctyl phthalate to plasticize the coating, andthis mixture was dissolved in methyl ethyl ketone/toluol to obtain asuitable solution coating. This solution coating was applied at 7 mg/inand the solvent portion of this coating was evaporated by force dryingthe coating at 300 F for 5 minutes. The resulting sheet was coated witha paraffin wax dissolved in hexane to obtain a wax weight of 20 mg/ft.The coated and waxed sheet was drawn, redrawn, sized and ironed withoutfurther heating or waxing to 211x408 beverage cans.

EXAMPLE 3 A sheet of Aluminum Alloy 3004-1-119, 0.0145" was cleaned andpretreated as in Examples 1 and 2 and subsequently coated with a vinylresin dispersion-type coating of the following composition.

Parts by Weight Components 100 Vinyl chloride dispersion resin (fineparticle size) Vinyl solution resin (86% vinyl chloride, 13% vinylacetate, 1% maleic acid) 45 Med MW epichlorohydrin-bisphenol resin 35Triazine resin 25 Non-reactive polymeric plasticizer 200 Titaniumdioxide pigment 600 Methylethyl ketone (MEK):toluol:xylol 11121 1Phosphoric acid catalyst 8 Paraffinic lubricant This coating compositionwas applied to one side of the sheet at 8 mg/in dry film weight, and thecoating was heated for 1 minute at 500 F oven temperature to facilitatesolvent evaporation and cross-linking of the reactiveportions of theresin solution. The coated sheet was then drawn into a cup, redrawn,sized and ironed into a 21 l 400 can without substantially disruptingthe continuity of the coating on the inside side wall.

EXAMPLE 4 A sheet of Aluminum Alloy 3004-H19, 0.0145" was cleaned andpretreated with a chromium-phosphate conversion coating having a filmweight of 25 mg/ft and subsequently coated with a high molecular weightnoncross-linked epoxy resin. The epoxy resin (epichlorin-bisphcnol type)had an average molecular weight of 36,000 and was dissolved in a mixtureof methyl ethyl ketone (MEK) and cellusolve acetate, the final coatinghaving a resin content of 18 per cent by weight and a solvent content of82 per cent by weight. The resin was was substantially free of pinholes.Adhesion was judged excellent as measured by scotch tape adhesion tests,and the coated metal surface withstood beer pasteurization of 20 minutesat 180 F in beer without loss of coating adhesion. Film weightmeasurements on the side wall were approximately 3 mg/in whereas filmweight on the bottom surface was about 9 mg/in.

EXAMPLE 5 3004 conversion-coated aluminum sheet, 0.0145" was coated witha solution polymer consisting of a high molecular weight epoxy resin ofthe epichlorohydrinbisphenol type as in Example 4 except that thepolymer was blended with by weight urea-formaldehyde resin (UformiteF240) to facilitate partial cross-linking. The resin was dissolved inMEK/butyl cellusolve to facilitate application. The coating was appliedat 8 mg/in and heated for 5 minutes at 425 F to facilitate solventevaporation and resin cross-linking. The resulting precoated sheet wasthen handled substantially as described in Example 3 with substantiallythe same results.

EXAMPLE 6 3004 alloy conversion-coated aluminum sheet was coated with asolution polymer consisting of a medium molecular weight epoxy resin(epichlorohydrinbisphenol type) modified with a urea-formaldehyde resinas in Example 3 except for modification to obtain a cross-linking of 70%of that theoretically available based on the equivalent weight of theepoxy and ureaformaldehyde resin. The resin was dissolved in MEK/-toluol/isopropyl alcohol, applied to a dry film weight of 7 mg/in andheated for 5 minutes at 400 F to facilitate solvent evaporation andcross-linking. A lubricant consisting of paraffin wax dissolved inhexane was applied to obtain a dry lubricant weight of mg/ft The sheetso prepared was drawn, redrawn, sized and ironed to obtain a 21 lX400beverage can. The coating was somewhat milky after ironing but adheredwell to the substrate.

EXAMPLE 7 To both sides of 3004-H19, 0.0145 pretreated aluminum sheetwas applied a solution vinyl coating as in Example 2. The sheet waslubricated and subsequently drawn and ironed to a 21 1X408 beverage can.Both inside and outside coating adhered well to the pretreated sheet.Film continuity and dry adhesion were'acceptable for commercial use.

It is believed apparent from the foregoing description and examples thatby my invention I have provided a method which permits economical use ofprecoated aluminum sheet in the manufacture of drawn and ironed beveragecans.

While the invention has been described in terms of preferredembodiments, the claims appended hereto are intended to encompass allembodiments which fall within the spirit of the invention.

Having thus described my invention and certain preferred embodimentsthereof, I claim:

1. In a process for forming shaped articles by drawing and ironingaluminum sheet or strip coated with an organic coating suitable forcontact with comestibles, the successive steps comprising:

1. coating a conversion coated aluminum sheet or strip of a gaugeadapted for drawing and ironing containers with a resin capable ofwithstanding substantial deformation in drawing and ironing operationswhile remaining integral without substantial fracture or exfoliationselected from the class consisting of a. vinyl resins having a vinylchloride content of from 81-97% by weight and a vinyl acetate content of3-14% by weight;

b. uncross-linked epoxy resins having a molecular weight of at least36,000 and c. epoxy resins cross-linked with amino or vinyl resins, saidepoxy resin having a minimum molecular weight of 5,000 and saidcross-linked resin having a maximum cross-linkage of 2. curing the resinthus coated on said sheet or strip;

3. drawing the thus coated sheet with resin cured thereon into the shapeof a shallow cup-shaped article and, thereafter, without first activelyheating the drawn article;

4. ironing said shallow cup-shaped article into the form of an ironedarticle.

2. The process of claim 1 wherein said aluminum sheet is coated with avinyl coating comprising vinyl chloride, vinyl acetate, and maleic acid.

3. The process of claim 1 wherein said aluminum sheet is coated with anuncross-linked epichlorohydrinbisphenol type epoxy resin.

4. The process of claim 1 wherein said aluminum sheet is coated with anepichlorohydrin-bisphenol type epoxy resin cross-linked with aurea-formaldehyde resin.

5. The process of claim 1 wherein said aluminum sheet is coated with anepichlorohydrin-bisphenol type epoxy resin cross-linked with a vinylresin.

6. The process of claim 1 wherein said aluminum sheet is drawn andironed from a gauge of at least about 0.0145 inch to a gauge of lessthan about 0.006 inch.

2. The process of claim 1 wherein said aluminum sheet is coated with a vinyl coating comprising vinyl chloride, vinyl acetate, and maleic acid.
 2. curing the resin thus coated on said sheet or strip;
 3. drawing the thus coated sheet with resin cured thereon into the shape of a shallow cup-shaped article and, thereafter, without first actively heating the drawn article;
 3. The process of claim 1 wherein said aluminum sheet is coated with an uncross-linked epichlorohydrin-bisphenol type epoxy resin.
 4. The process of claim 1 wherein said aluminum sheet is coated with an epichlorohydrin-bisphenol type epoxy resin cross-linked with a urea-formaldehyde resin.
 4. ironing said shallow cup-shaped article into the form of an ironed article.
 5. The process of claim 1 wherein said aluminum sheet is coated with an epichlorohydrin-bisphenol type epoxy resin cross-linked with a vinyl resin.
 6. The process of claim 1 wherein said aluminum sheet is drawn and ironed from a gauge of at least about 0.0145 inch to a gauge of less than about 0.006 inch. 